Don't let its toyish charm fool you. These are the ultimate live gear tool if you want to travel light. The cartridge is a little rare but found easily online. There is also the non-plus version which is easier to find. However, the plus version allows dual mode (so you can play four synth lines at once). A fun piece of gear especially if you already have a DS, DSi, DSiXL. I'm going to pair this up with my old school Gameboy/Camera music program before getting another DS.

There is also the Korg M01 too! Japan only but works with US DS. Cost is steep though since it's an import.